Marine jet propulsion arrangement

ABSTRACT

A jet propulsion arrangement, including a jet propulsion unit ( 32 ) having a housing ( 38 ) which includes an inlet ( 40 ) through which liquid is received, an outlet ( 34 ) through which liquid is discharged, a liquid path ( 39 ) between the inlet ( 40 ) and the outlet ( 34 ) and an impeller ( 41 ) disposed in the liquid path ( 39 ) to drive liquid for discharge. The housing can define a gear chamber ( 45 ) which accommodates a gear arrangement ( 46 ), with a first drive shaft ( 44 ) extending from the gear arrangement ( 46 ) in direct connection to the impeller ( 41 ) and a second drive shaft ( 54 ) extending from the gear arrangement ( 46 ) to an engine ( 31 ). Alternatively, a single shaft can extend in direct connection between the impeller ( 41 ) and the engine ( 31 ).

The present invention relates to a marine jet propulsion arrangement for use in marine vessels, such as pleasure boats, ski boats, commercial and semi-commercial boats.

Jet propulsion in marine vessels provides an alternative to propulsion by propeller drive and is generally recognised as providing a range of advantages compared to the use of propellers. These advantages include:

-   -   a greater degree of safety to any person in the immediate         vicinity of a vessel employing the jet drive because there is no         exposed rotating propeller,     -   the intake for the jet is normally flush with the hull of the         vessel, so that there is minimised draft compared to a propeller         drive which extends downwardly of the hull,     -   superior manoeuvring, including the ability to move the vessel         sideways if a pair of jets are employed,     -   superior stopping, whereby a vessel can be stopped within         several boat lengths from planing speed     -   lower long term maintenance,     -   higher output efficiency,     -   longer engine life,     -   lower drag,     -   smoother and quieter ride,     -   environmentally more friendly.

A major disadvantage with the use of jet propulsion units, is the necessity for a complicated drive train between the engine of the vessel and the jet propulsion unit. To date, a vessel which employs a jet propulsion unit typically would include a gearbox connected to the engine, a drive shaft connected to the gearbox at one end and at the other end to the jet propulsion unit. The connections at each end of the drive shaft are of the flexible coupling type and the overall arrangement is generally cumbersome to install and maintain, as well as being expensive and bulky. The cumbersome nature of the arrangement is compounded by the fact that each of the engine, gearbox, drive shaft, couplings and jet propulsion unit, are manufactured by different manufacturers, and that requires those manufacturers to produce their particular component to correspond for connection and loading purposes, with the components of the other manufacturers. Additionally, maintenance of the drive train requires the sourcing of the components from the different manufacturers, which complicates maintenance procedures. It also creates difficulties in respect of warranties, because the component suppliers will typically blame each other in the event of failure of one or more of the components, so that ultimately, any warranties are difficult to claim. A further disadvantage is that present jet propulsion arrangements encroach significantly further into the interior of a vessel, when compared to similar horsepower sterndrive arrangements.

Despite the numerous advantages provided by jet propulsion, the associated disadvantages have to date outweighed the advantages, to the extent that the use of jet propulsion to date has been very limited and mainly restricted to larger commercial vessels. Given the significant advantages inherent in jet propulsion, it is an object of the present invention to provide a jet propulsion arrangement that facilitates more general use of jet propulsion for driving marine vessels.

According to one aspect of the present invention there is provided a jet propulsion arrangement, including a jet propulsion unit having a housing which includes an inlet through which liquid is received at a first end thereof, an outlet through which liquid is discharged at a second end thereof, a liquid path between said inlet and said outlet and an impeller disposed in said liquid path to drive liquid for discharge, said housing defining a gear chamber generally in the region above said inlet which accommodates a gear arrangement, a first drive shaft extends from said gear arrangement in direct connection to said impeller and a second drive shaft extends from said gear arrangement to an inboard engine said first and second drive shafts extending substantially parallel to each other and in substantially opposite directions.

The present invention is distinguished in a first aspect from the prior art by the integration of a gear arrangement within the housing of the jet propulsion unit, as well as by the direct connection of the unit to the engine through the second drive shaft. In this manner, the jet propulsion arrangement can have a single manufacturer, including each of the drive shafts and the gear arrangement and the second drive shaft can be arranged or designed for direct coupling to a suitable marine engine, so that the installation of the jet propulsion arrangement is simplified. That simplification can be such that the jet propulsion arrangement can be installed with a single connection through the second drive shaft to the engine and by mounting the arrangement in the normal manner to the hull of the vessel.

According to a further aspect of the present invention, there is a jet propulsion arrangement including a jet propulsion unit having a housing which includes an inlet through which liquid is received at a first end thereof, an outlet through which liquid is discharged at a second end thereof, a liquid path between said inlet and said outlet and an impeller disposed in said liquid path to drive liquid for discharge, a drive shaft extending from said impeller for connection to an engine, said engine having a rear end region and a forward end region and said rear end region being closer to said jet propulsion unit than said forward end region, said engine being supported at each of said forward and rear end regions and said rear end region support being provided by said connection to said jet propulsion unit.

According to each aspect of the present invention, the arrangement also can have a much more compact length, as well as a significant weight reduction. A significant contributor to weight reduction is by the reduction in necessary drive shafts. In the prior art, at least four drive shafts are employed, comprising:

-   -   1. an impeller drive shaft within the jet propulsion unit         extending from the impeller,     -   2. an external drive shaft, extending from the impeller drive         shaft to the gearbox,     -   3. a first gearbox drive shaft, extending from the gearbox to         the external drive shaft, and     -   4. a second gearbox drive shaft, extending from the gearbox to         the engine.

In contrast, the present invention in one aspect requires only two drive shafts, which not only eliminates two of the required prior art drive shafts, but also a significant number of associated bearings and seals, while the simplification of the drive train means it is more easily balanced for smoother ride and there is less opportunity for component failure. Further, in another aspect of the invention, there is a requirement for a single drive shaft only, for direct drive between an engine and the impeller of the jet propulsion unit. A further significant advantage is in lower cost by the elimination of various components.

The second drive shaft can be customised to suit existing engine couplings, so that there is no modification or adaptor required for the invention to be coupled to an existing engine. In particular, the second drive shaft may be arranged for coupling to an engine which is presently designed for coupling to a propeller drive assembly of the kind known as a “sterndrive”. This advantageously eliminates the necessity for redesign of the couplings for such engines.

The present invention has been designed principally for coupling to marine engines presently employed in sterndrive arrangements, and in particular to such engines known by the registered trade mark MerCruiser, manufactured the US company Mercury Marine. However, it is to be appreciated that the second drive shaft could easily be configured for coupling to alternative engines, for example larger engines such as those employed in marine vessels of the commercial or semi-commercial kind.

The jet propulsion unit of the invention preferably is generally elongate, with the outlet at one end thereof and the gear chamber at the opposite end. The gear chamber is preferably positioned in an upper portion of the housing when the jet propulsion unit is installed in a vessel and preferably the chamber position is approximately above the housing inlet.

The gear arrangement can take any suitable form and in one form, it can comprise a pair of meshing helical gears which provide for rotational speed reduction between the second and first drive shafts. Alternately, the gear arrangement could take other forms as suitable, such as a planetary gear assembly. Preferably the chosen gear arrangement permits the first and second drive shafts to extend along parallel axes, with the axis of the first drive shaft being spaced below the second drive shaft when the jet propulsion arrangement is installed.

The gear chamber can be formed as an integral part of the housing, or it can be connected thereto in any appropriate manner. If the housing is completely cast, then the gear chamber may be formed as part of that casting. In one arrangement, the housing is cast in at least two sections which are bolted together. In another arrangement, a part or parts of the housing is cast, while another part or parts is fabricated from sheet metal. In any of these arrangements, the gear chamber may be cast or fabricated as appropriate. For weight efficiency, the casting material is preferably an aluminium alloy, such as ASTM-A356.

The invention conveniently can be arranged so that the jet propulsion unit provides rear end support for an engine. Such an arrangement advantageously allows a standardised system of attaching a range of different engines to a single capacity jet propulsion arrangement. Moreover, this minimises or completely eliminates, the need for any change or modification to existing arrangements which employ a stern drive system. Accordingly, the jet propulsion arrangement can replace an existing stern drive arrangement, or can be adopted initially rather than adopting a stern drive arrangement, without requiring modifications to the existing structure of the vessel and its engine mounting.

In the above arrangement, a marine engine has a forward end region and a rear end region, and the rear end region is adjacent the jet propulsion arrangement. The mounting arrangement of the engine is such that it is supported in the marine vessel at the forward end region, preferably at two mounting points respectively on either side of the engine, and is further supported by its coupling or connection to the jet propulsion arrangement at the rear end region thereof.

Embodiments of the invention are described in detail in the following passages of the specification which refer to the accompanying drawings. The drawings, however, are merely illustrative of how the invention might be put into effect, so that the specific form and arrangement of the various features as shown is not to be understood as limiting on the invention.

In the drawings:

FIG. 1 shows a prior art jet propulsion arrangement.

FIG. 2 shows a prior art sterndrive arrangement.

FIG. 3 is a side view of a jet propulsion arrangement according to the invention.

FIG. 4 is a side view of a jet propulsion unit according to the invention.

FIG. 5 is a cross-sectional view taken through V-V of FIG. 4.

FIG. 6 is a cross-sectional view of an alternative arrangement to that shown in FIG. 5.

Referring to FIG. 1, this shows a jet propulsion arrangement 10 fitted within a marine vessel V, which includes an engine 11, nominally a MerCruiser™ 1.7 L diesel, to which is bolted a gearbox 12. As will be appreciated, a drive connection in the form of a drive shaft extends between the engine 11 and the gearbox 12. Each of the engine 11 and the gearbox 12 would typically be manufactured by different companies. The connection type between the engine 11 and the gearbox 12 would normally be a bolted connection, although this is not shown in FIG. 1.

The output shaft 13 of the gearbox 12 has a flexible coupling 14 for coupling to a drive shaft 15. The drive shaft 15 is formed with yoke type connectors 16 at each end thereof and at the opposite end to the output shaft 13, the drive shaft 15 connects to the input shaft 17 of a jet propulsion unit 18. Like the engine 11 and the gearbox 12, the drive shaft 15 and the jet propulsion unit 18 each would typically be manufactured by different companies.

FIG. 2 shows a prior art sterndrive arrangement 20, fitted to a marine vessel V. The arrangement 20 includes a sterndrive 21 connected to an engine 22 which is of the same type as the engine 11 of FIG. 1. It will be appreciated from a comparison between FIGS. 1 and 2, that the sterndrive arrangement 20 is a much simpler arrangement than the jet propulsion arrangement 10. In particular, the sterndrive arrangement 20 requires only the two elements, namely the sterndrive 21 and the engine 22, whereas the jet propulsion arrangement 10 includes the additional elements of the gearbox 12 and the drive shaft 15 as well as additional componentry not shown. The added complexity associated with the jet propulsion arrangement has to date precluded the general adoption of such arrangements over the commonly adopted sterndrive arrangement. The present invention attends to this by a unique and simplified arrangement.

Referring to FIG. 3, a jet propulsion arrangement 30 according to the present invention is shown. The arrangement 30 is fitted to a marine vessel V of the same kind shown in FIGS. 1 and 2, and includes an engine 31 which is of the same type as the engines 11 and 22 of FIGS. 1 and 2. Illustration of the same engine is deliberate, in order to highlight the manner in which the present invention can be adapted, advantageously for use with existing engines, in particular those employed in sterndrive arrangements. The arrangement 30 includes a jet propulsion unit 32, which has similar characteristics at the rear or outlet end 33 thereof as the same end of the jet propulsion unit 18 of FIG. 1. However, the jet propulsion unit 32 is constructed very differently at the other or opposite end thereof, as will be explained in relation also to FIGS. 4 and 5.

Referring to FIG. 4, the jet propulsion unit 32 of FIG. 3 is shown separate from the engine 31. The unit 32 has an outlet end 33, which includes a discharge nozzle 34 and a steering nozzle 35. The steering nozzle 35 is mounted for rotation about a vertical axis A for left to right steering. A hydraulic mechanism 36 is provided for nozzle trim.

The unit 32 also includes a reverse “bucket” 37 which is shown in solid outline in an upward position and in broken outline in a downward position. In the upward position, a vessel to which the unit 32 is fitted will move forward, while in the downward position, the vessel can be reversed by reversing the discharge direction of the liquid jet. Again, because this aspect of the unit 32 forms no part of the present invention, further operational detail will be omitted.

The unit 32 includes a housing 38 which defines a passage 39 (see FIG. 5) extending between a liquid inlet 40 and the discharge nozzle 34. Disposed between the inlet 40 and the discharge nozzle 34 is an impeller 41 (see FIG. 5) comprising a rotor 42 and a stator 43. The rotor 42 is driven by a first drive shaft 44, which extends through the passage 39 into a chamber 45 which accommodates a gear arrangement 46.

The gear arrangement 46 includes a pair of meshing helical gears 47 and 48, which are mounted in vertical spacing, although that is not essential. The gear 0.47 is fixedly connected to the drive shaft 44, while the gear 48 is flexibly connected to a splined shaft 49. The flexible connection, which is not shown, is nevertheless disposed between the facing plate members 50 and 51 and accommodates slight misalignment between the engine to which splined shaft 49 is connected and the axis 52 of the gear 48.

The shaft 49 advantageously can be of the same type that extends from known sterndrive arrangements, and this is the case as shown in FIGS. 2 and 3. Thus, there is no necessity for engine modification, or the use of an intermediate adaptor, as the invention provides for direct coupling of the jet propulsion unit to the engine in the same manner as the existing sterndrive arrangements.

Each of the gears 47 and 48 is mounted to the housing on shafts 53,54 which rotate within suitable bearing arrangements as shown and the connection between the gears and the shafts can be by any suitable arrangement such as splined or keyed.

To close the gear chamber 45, a cover plate 55 is bolted to the housing 38 and it can be seen that the splined shaft 49 extends through the plate 55. It will be appreciated that a suitable sealing arrangement will be employed so that lubricant within the chamber 45 will not leak through the opening through which the shaft 49 extends. Likewise, the drive shaft 44 extends through an opening in the wall of the housing 38, from the chamber 45 to the impeller 41 and a suitable sealing arrangement is also employed at that opening in the wall of the housing 38.

Referring to FIGS. 3 and 4, the unit 32 further includes an inspection port 56 for inspecting the interior of the unit 32 when it is installed, and a power assisted reverse mechanism 57, which is driven from the engine 31 (FIG. 3). The shaft 54 (FIG. 5) drives a pulley 58 and belt (not shown) to drive, say a hydraulic pump or a water pump.

The gear arrangement 46 of FIG. 5 provides a relatively uncomplicated arrangement for transmitting engine torque to the impeller 41. However this arrangement has encountered difficulties with the drive provided by certain engines, particularly at idle or very low engine speeds. The problem resides in the torque fluctuations at those engine speeds, which are largely absorbed through the mesh between the gears 46 and 47, and that results in gear clatter and increased vibration noise. Certain types of engines experience greater problematic torque fluctuations than others and therefore the invention extends to an alternative arrangement to damp the torque fluctuations and therefore reduce vibration noise and gear clatter and to thereby improve driveline life.

In the alternative arrangement, the splined shaft arrangement of FIG. 5, comprising the splined shaft 49, and the facing plate members 50 and 51, is removed and replaced with the arrangement 100 illustrated in FIG. 6. In that figure, like parts from FIG. 5 have the same reference numeral, plus 100.

In the arrangement 100, the shaft 154 has attached to one end thereof, a socket 160, which is attached to the shaft 154 by a bolt 161. The socket 160 defines an inner toothed surface which meshes at 162 with an outer toothed surface of a female coupling member 163. In the toothed connection 162 between the socket 160 and the coupling member 163, the teeth of the member 163 are crowned, so that the member 163 is allowed a slight pivoting movement about the centre of the crowned tooth profile, for accommodating misalignment between the socket 160 and the member 163.

The coupling member 163 is connected by way of the shaft 164 to a dual stage damping arrangement, which includes a drive plate 165 for connection to the flywheel of a suitable engine, such as the engine 31 of FIG. 3.

The drive plate 165 includes a plurality of pins 166, although only a single pin is shown in FIG. 6. Preferably three or four pins are provided. Each pin includes a resilient, preferably rubber sleeve 167 and a metal sleeve 168. Mounted centrally of the drive plate 165 is a stub shaft 169 which has a step 170 to locate an inner race of a spherical-type bearing 171. The bearing 171 seats against the internal surface of a housing 172 between a step at one end and a circlip at the other. The inner race of the bearing 171 is engaged at the end remote from the step 170 by a sleeve portion 173 of a clamping member 174. The clamping member 174 is fixed to the stub shaft 169 by an arrangement including plates 175 and 176 and by bolts 177 and 178. The bolt 177 is one of preferably three or four such bolts which fasten each of the clamping member 174 and the plates 175 and 176 together, while the bolt 178 secures that assembly to the stub shaft 169. The bearing 171 is therefore firmly captured between the stub shaft 169 and the housing 172 and by the arrangement of the sleeves 167 and 168 on the pins 166, and the bearing 171, angular misalignment at the end of the shaft 164 opposite the coupling member 163 is accommodated.

The housing 172 includes a slot to accommodate each of the pins 166 and each slot is formed arcuately curved to allow pin movement within the slot relative to the housing 172 so that the drive plate 165 can rotate relative to the housing 172. The arcuate length of each slot is equal and is arranged to allow an amount of movement of the pins 166 sufficient to provide suitable damping as will be explained later. When the pins have reached an end of the slot in which they are captured, then the drive plate 165 and the housing 172 will rotate together. This can occur in either direction of rotation.

A torsional arrangement 179 cooperates with the housing 172 and includes the plate 176, to govern relative rotation between the drive plate 165 and the housing 172 and further includes an axial member 180 and a plate 181. The torsional arrangement 179 is formed of the plate 176, the axial member 180 and the plate 181, which components are connected together by a vulcanised rubber material so that the respective parts are torsionally flexible, with resistance to relative movement between the parts increasing as the relative movement occurs. The torsional arrangement 179 is a component which can be bought commercially, such as through the manufacturing company Lord which specialises in products of this kind. The torsional arrangement is such that, with the pins 166 centred within their respective slots, relative rotation between the drive plate 165 and the housing 172 will occur only when there is a torque input from the flywheel of the engine which varies or fluctuates, either increasing or decreasing.

The arrangement of FIG. 6 advantageously is operable to accommodate misalignment between the arrangement 100 and the shaft 154 as discussed above, and is further operable to damp vibration noise and gear clatter at engine idle and low speeds. That is, by allowing relative movement between the drive place 165 and the housing 172, torque fluctuations at engine idle and low speed can be damped, so that a more even torque is transmitted to the gears 146 and 147, thereby reducing noise and gear clatter and improving overall driveline life.

Returning to FIGS. 4 and 5, these also show the shaft 49 extending adjacent fingers or projections 60. In each figure, only a single projection is shown, and each figure shows the projection on an opposite side of the shaft 49. Each of the projections 60 includes a bore 61 having an axis 62. The projections 60 are employed for mounting of the engine 31 (FIG. 3) by way of a bracket (not shown) that extends from the engine 31 to over or underly the projections 60 and which includes bores for alignment with the bores 61, so that suitable bolt fasteners can fix the engine to the projections 60.

The projections 60 extend from the cover plate 55, which is arranged to have limited rotation so that any misalignment between the engine and the jet propulsion unit 32 can be accommodated. The cover plate 55 is intended to be fixed in place, rather than to the continuously rotatable, but fixing takes place when the cover plate has been rotated to an appropriate position. With reference to FIG. 5, a through bore 63 is shown extending through the cover plate 55 and through the housing 38, through which bolt 64 (FIG. 4) extends and is fastened. A further bolt 65 completes the connection of the cover plate 55 to the housing 38. For the cover plate 55 to rotate, the part of the bore 63 that extends through either the cover plate 55 or through the housing 38, can be an arcuate slot. A similar arrangement can be applied in relation to the bolt 65.

The above arrangement advantageously can accommodate misalignment between the jet propulsion unit 32 and the engine, but its other major advantage relates the use of twin jet propulsion units in larger marine craft. These craft employ twin engines to drive the jets, but the jets are mounted directly on the hull of the craft, or either side of the apex of the hull, so that they are mounted on the angle of the hull. The engines however cannot be mounted on an angle but rather are mounted above the hull horizontally. Therefore the rotational mount provided by the present invention facilitates connection between a horizontally mounted engine and an angled jet. The connection arrangement is simple, as it is necessary just to connect the jet to the engine by suitably rotating the cover plate 55 and then to fix the cover plate 55 in the angled position by the bolts 64 and 54.

It will be readily appreciated that the present invention provides distinct installation advantages over the prior art. A major advantage is that the jet propulsion unit 32 is a single unit ready for mounting within a hull and for direct connection to an engine. A comparison between the arrangement of FIGS. 1 and 3 shows the invention to be of a more compact form that the prior art, for comparable jet unit and engine size, while the invention, by the reduction in parts, is lighter in weight and of reduced cost. Other advantages of the invention have been expressed earlier herein. Accordingly, the invention is likely to facilitate the adoption of jet propulsion for marine vessel drive, with all the associated benefits, not only for smaller vessels, but for larger vessels as well.

Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention. 

1. A jet propulsion arrangement, including a jet propulsion unit having a housing which includes an inlet through which liquid is received at a first end thereof, an outlet through which liquid is discharged at a second end thereof, a liquid path between said inlet and said outlet and an impeller disposed in said liquid path to drive liquid for discharge, said housing defining a gear chamber generally in the region above said inlet which accommodates a gear arrangement, a first drive shaft extends from said gear arrangement in direct connection to said impeller and a second drive shaft extends from said gear arrangement to an inboard engine said first and second drive shafts extending substantially parallel to each other and in substantially opposite directions.
 2. A jet propulsion arrangement according to claim 1, said jet propulsion unit being generally elongate, with said outlet disposed at one end thereof and said gear chamber disposed at an opposite end.
 3. A jet propulsion arrangement according to claim 1, said liquid path being defined by housing walls which form a conduit that extends between said inlet and said outlet, wherein said first drive shaft extends through a housing wall of said conduit and into said liquid path for connection to said impeller.
 4. A jet propulsion arrangement according to claim 1, wherein said first drive shaft extends substantially horizontally between said gear arrangement and said impeller.
 5. A jet propulsion arrangement according to claim 1, wherein the axis of rotation of said first drive shaft is below the axis of rotation of said second drive shaft.
 6. A jet propulsion arrangement according to claim 1, wherein said gear arrangement includes a pair of meshing helical gears which provide a rotational speed reduction between the first and second drive shafts.
 7. A jet propulsion arrangement according to claim 6, said helical gears being in substantially vertically spaced meshing engagement.
 8. A jet propulsion arrangement according to claim 6, a first of said helical gears being fixedly connected to an end of said first drive shaft, and in second of said helical gears being flexibly connected to an end of said second drive shaft, said flexible connection accommodating axial misalignment between said engine and said gear arrangement.
 9. A jet propulsion arrangement according to claim 1, wherein said gear chamber is formed as an integral part of said housing, and defines an opening for access to within said chamber which is closed by a removable plate.
 10. A jet propulsion arrangement according to claim 1, wherein said housing is cast in at least two sections which are subsequently connected together.
 11. A jet propulsion arrangement according to claim 1, wherein said gear chamber is separately connected to said housing.
 12. A jet propulsion unit for use in a jet propulsion arrangement according to claim
 1. 13. A jet propulsion arrangement according to claim 1 wherein said arrangement includes a couple for connection to the inboard engine said couple including a pair of projections which extend from a plate through which said second drive shaft extends and each said projection includes a bore, said projections being arranged to overly or underly complementary mounting projections of the inboard engine, said mounting projections each including a bore and said respective bores being arranged for alignment and to accept a fastener for fastening the respective projections of said couple and the inboard engine together.
 14. A jet propulsion arrangement according to claim 13 said pair of projections of said couple extending from opposite sides of the said second drive shaft.
 15. A jet propulsion arrangement according to claim 13, wherein said couple is rotational so that the arrangement can be coupled to the inboard engine at an angle.
 16. A jet propulsion arrangement according to claim 15, said rotational couple having an arc of rotation of about 45°.
 17. A jet propulsion arrangement according to claim 1, said engine having a rear end region and a forward end region and said rear end region being closer to said jet propulsion unit than said forward end region, said engine being supported at each of said forward and rear end regions and said rear end region support being provided by connection between said engine and said jet propulsion unit.
 18. A jet propulsion arrangement including a jet propulsion unit having a housing which includes an inlet through which liquid is received at a first end thereof, an outlet through which liquid is discharged at a second end thereof, a liquid path between said inlet and said outlet and an impeller disposed in said liquid path to drive liquid for discharge, a drive shaft extending from said impeller for connection to an engine, said engine having a rear end region and a forward end region and said rear end region being closer to said jet propulsion unit than said forward end region, said engine being supported at each of said forward and rear end regions and said rear end region support being provided by said connection to said jet propulsion unit.
 19. A jet propulsion unit having a housing which includes an inlet through which liquid is received at a first end thereof, an outlet through which liquid is discharged at a second end thereof, a liquid path between said inlet and said outlet and an impeller disposed in said liquid path to drive liquid for discharge, said housing defining a gear chamber generally I the region above said inlet which accommodates a gear arrangement, a first drive shaft extends from said gear arrangement in direct connection to said impeller and a second drive shaft extends from said gear arrangement for connection to an inboard engine said first and second drive shafts extending substantially parallel to each other and in substantially opposite directions.
 20. A marine vessel including an engine and a jet propulsion unit according to claim
 19. 